Sensor for verification of genuineness of security paper

ABSTRACT

A method and a device for automatic verification of genuineness of a banknote or a document comprising a watermark is described. A two-part, doubly active capacitive sensor device (4, 6, 7) is used. A symmetry property of the sensor output signal is changed in a predetermined manner when a correct watermark is present in a coinciding position with shape-adapted capacitor electrodes (4, 6).

The present invention concerns recognition and approval or rejection ofa watermark in a paper note or a document. The pattern of the watermarkmust comprise a special feature, namely that it consists of twocharacteristically shaped neighbouring areas, whose thicknesses differin being both thicker and thinner than the average thickness of the notein the watermark region, while the words, area density (mass per unitarea) and thickness are variable quantities, while mass density isconstant. This as opposed to a usual form of counterfeit watermark,which is made by pressing the sheet together in order to give a variablethickness. In this case mass density and thickness will vary in aninverse relationship, while area density stays constant. A genuinewatermark is formed by "thickness modulation" during the paperproduction process, so that mass density of the paper stays constant.

If the paper note is equipped with an implanted security thread forverification of genuineness, this thread may also serve as a usable testobject in a variant of the present invention. Such a security thread mayconsist of metal, metallized plastics, plastics of a similar material.

There has for quite some time existed a need of a fast and reliablemethod of verification of genuineness of banknotes and documents inconnection with the banknote testing in national banks, and also in asmaller scale, for instance in banknote operated vending machines.

There has been made attempts to solve this problem by the use of opticaltechniques, but modern copying engineering is capable of fooling most ofthe optical detection methods. The watermark is still regarded to be anadequate and safe way of marking a genuine note, and a mechanicalmeasurement of thickness has previously been used in testing watermarks.However, this technique is not well suited to a rapid machine procedure,and is not very useful when the note has small injuries distributed atrandom. Besides, the thickness modulation of a watermark may beinitiated relatively simply as explained above.

However, Swedish laid-open publication No. 355,428 discloses a measuringtechnique which is based upon the fact that the capacitance of an airplate capacitor is changed when for instance a paper note is pushed intothe air space between the electrode plates. The paper thickness, orrather the area density of the paper, is related to the capacitance thatis sensed. A specially designed capacitor is used, in which one of theelectrodes has the same shape as for example a thickened part of thesought watermark. A dynamic measurement of capacitance is made while thenote is led through the capacitor. If a correct watermark passes theadjusted electrode, capacitance will increase abruptly before anddecrease equally abruptly after a maximum which is reached just atcoincidence. The graph showing the capacitance change (as a function oftime or position of the note) should have a special appearance to beapproved according to particular condition, or else rejected. TheSwedish publication also hints at the possibility of making a doublesuch analysis, first one for a thickened pattern, and thereafter one fora thinned pattern, which will usually belong to the same watermark.

The capacitive sensor device mentioned above suffers, however, from afew drawbacks or weaknesses:

Firstly, this device is unable to see the difference between thin andthick paper sheets. The reason for this is that the measurement has adynamic character and only detects the change in capacitance as thewatermark passes the sensor. A signal indicating absolute thickness ofthe paper will therefore not appear, only one indicating only oneindicating changes of thickness. Thus paper quality cannot beinvestigated while the note is passing. Nor will a double or possiblymultiple paper feeding, with a number of paper simultaneously, bedetected by this device.

Electrically both the capacitor electrodes of the known sensor deviceare arranged "floating" relative to ground, which entails problemsconcerning stability and influence by external electromagnetic fields.

The most important weakness about the known device is, however, that thedynamic measuring principle which is used, implies that the sensordevice may be fooled by for example a hole in the watermark region,which may be interpreted as an acceptable watermark. It is supposed thatthis must be a main reason why the mentioned sensor device has notachieved a wide recognition, or has been put into use by a majority ofmanufacturers of vending machines or note testing machines.

Additionally, the prior art sensor device seems to have an unnecessarilycomplicated structure, and it must be constructed as a double device inorder to test a normal watermark, which has both thinned and thickenedparts.

Using the method and the apparatus according to the present invention,it is achieved that a genuine watermark will be recognized, while acounterfeit, imprinted imitation mark will produce a deviating signal.It is further achieved that only a correctly designed watermark willyield a recognition signal, while holes in the paper or other,differently formed thickness modulations of the paper will be easilydetected. (A hole shall for example entail a capacitance measurementwhich deviates in both positive and negative directions when the hole'sedges are in the sensor area, contrary to the prior art device, which isonly able to give a positive signal when there is a change incapacitance value.) Besides, an absolute measurement of the paperthickness or quality may be brought about. Such an absolute thicknessmeasurement also gives the apparatus of the invention the advantage thatthe occurrence of double feeding or possibly several paper notes on topof each other, is measure just like a correspondingly thicker paper, andsuch an occurrence may consequently be pointed out in a simple manner.This is a feature which may be useful in many instances. Additionally,one rapidly and simply achieves a measurement which comprises both thickand thin parts of a watermark. An implanted metal thread may also berecognized.

These and other advantages are obtained by a method for approving abanknote or a document with a watermark, the pattern of said watermarkconsisting of two characteristically shaped neighbouring areas with alocal area density (mass per unit area) which is markedly higher resp.lower than the principal average area density of said note in thewatermark region, the method being characterized in that said watermarkof said banknote or document, or characteristic sections thereof, isbrought to a position corresponding with a two-part, doubly activecapacitive sensor device, which sensor device consists of a common, flatmetal plate as one capacitor side, which metal plate may be connected toground, said sensor device at the other capacitor side being dividedinto two metal plates situated both in the same plane, said two platesbeing adapted in shape to each one of said two characteristically shapedneighbouring areas or characteristic sections thereof and beingelectrically separated, however with insignificant separation distancecompared to the other areawise dimensions of said two plates, whereby apreset symmetry property of the double output signal from said sensordevice is disturbed in a predetermined manner when a correct watermarkcoincides with the two sensor plates, which symmetry property iscontinuously monitored by signal processing equipment connected to saidsensor device, which method also appears from patent claim 1 below.

Further advantages are attained using a method and a device as stated inthe additional claims.

In some cases the paper thickness may exhibit relatively strongvariations, distributed at random over the area of the note. It may beadvantageous then to use only a part of the watermark instead of thewhole, to achieve greater safety against influence on the measurementfrom these random variations of thickness. It is possible to select a"characteristic section" of the watermark, observing that this sectionincludes both thickened and thinned areas of the watermark. This part ofthe watermark should obviously not be made too small sincecharacteristic features of the watermark pattern then will disappear,and also the measurement signal (capacitance) will be too small.

A "two-part, doubly active capacitive sensor" is primarily intended tomean a capacitor of plate type with air as a dielectric, one capacitorside having a metal electrode plate which has been cut into two parts,and where the two parts are used in a quite equivalent manner inmeasuring capacitance against the single, common electrode platesituated on the other capacitor side. This is quite distinct from a caseas disclosed for example in the previously mentioned Swedish laid-openpublication No. 355.428, where a two-part capacitor plate occurs, butonly one central part is active in the sense of "measuring capacitance",while other outer part serves to guide the electrical field lines, i.e.it is a so-called "guard ring".

The invention will now be described closer, referring to the encloseddrawings, where

FIG. 1 shows part of a paper note including an imagined genuinewatermark,

FIG. 2 shows an upper, double capacitor plate constructed according tothe invention to detect the imagined watermark,

FIG. 3 shows all of the two-part capacitor according to the invention,with the upper and lower plate in a sidewise view,

FIG. 4 shows an example of an electrical signal processing circuit inaccordance with the invention, including the two-part capacitor,

FIG. 5 shows one particular shape of the output signal from a section ofthe signal processing circuit of FIG. 4,

FIG. 6 shows another example of an electrical signal processing circuitin accordance with the invention, and

FIG. 7 shows one shape of output signals from parts of the signalprocessing circuit of FIG. 6.

FIG. 1 shows part of a paper note 1 comprising a genuine watermark 2a,2b with a particular picturewise design, in this case two concentriccircular areas 2a and 2b. Generally the watermark may of course have amuch more complicated design, but a circular shape has been selectedhere for simplicity.

The watermark has been formed in the paper production process, andconsists of one thick area 2a with thickness T+ΔT and one thinned area2b with thickness T-ΔT, the paper having an average thickness of Taround the watermark. Local mass density is mainly constant all over thepaper, which paper is manufactured to be homogenous. Thus local areadensity, i.e. mass per unit area, is increased in the thick area 2a,while local area density is low in area 2b.

As opposed hereto, it must be remarked that a paper carrying animprinted pattern of the same design, shows a variable mass density andconstant area density.

It is an empirical fact that an imprinted (that is counterfeit) mark, inspite of thickness variation of a correct character, gives a practicallyconstant capacitance when led in between two capacitor plates, owing tothe constant area density. On the contrary, a genuine watermark havingvariable area density gives a variable capacitance contribution, whichis proportional to area density and easily detectable.

FIG. 2 shows the two-part electrode plate of the capacitor. As anexample the plate may consist of a glass fiber print board 3 with apattern etched in metal, preferably copper, the pattern being adapted inshape to the pattern shown in FIG. 1. An inner circular area 6 of copperhas substantially the same diameter as area 2a. An outer ring 4 ofcopper has mainly the same measures as area 2b. The circular area 6 andthe annular area 4 are separated by a small spacing 5. As an example thewidth of the spacing 5 may be 0.1 mm for diameters of 10.0 mm and 14.3nm: respectively belonging to inner circular area 6 and outercircumference of area 4. (These diameters give equal areas for the twoparts, which may be practical, however not necessary.)

In FIG. 3 the glass fiber print board 3 is found again, with copperareas 4 and 6 constituting one capacitor side of the two-part capacitorwhich is seen in a side view. The opposite capacitor side has one commoncopper electrode 7 situated on a glass fiber board 8. Electricalconductors are shown schematically at 9, 10 and 11, however, theseshould be made as short as possible. The distance d between thecapacitor plates is selected appropriately in relation to the maximumallowable paper thickness, for example a distance d equal to about 0.2mm. An example of a well suited signal processing circuit for therecognition of a correct watermark is shown in FIG. 4. The two-partcapacitors which are constituted by area 4 and common electrode 7, andarea 6 and common electrode 7, are represented in FIG. 4 by thecapacitances C₄ and C₆ respectively. Suitable resistances R₄ and R₆,together with said capacitances, provide a components determining timeconstants in order to define the durations T₄ and T₆ of the unstablestates of each component respective of two so-called "oneshot"multivibrators 12 and 13, which are mutually interconnected. An outputsignal U_(ut) which may be outputted from one of the multivibrators,will vary as shown in FIG. 5. The signal is a typical square signal witha rapid change between two constant voltage levels. The times duringwhich the signal stays in each of the levels between changes, arerespectively T₄ and T₆.

With an appropriate choice of parameter magnitudes, i.e. size ofelectrode areas 4 and 6, as well as resistance values of resistors R₄and R₆, T₄ and T₆ may for example be given equal duration when a paperwithout a watermark, that is with an even thickness, is put into thecapacitors. In this case the output signal U_(ut) will be a symmetricalsquare signal, T₄ being equal to T₆. As soon as the two capacitances C₄and C₆ change their values each in a different direction, a pronounceddeviation of the symmetry of the square signal is obtained, for instanceinto a shape like that shown in FIG. 5, where T₄ and T₆ are unequal.

As long as U_(ut) is symmetrical, its average value is situated halfwaybetween the two voltage levels, for example at 0 volts. With anon-symmetrical signal owing to imbalance between the capacitance valuesC₄ and C₆, a deviating average value is obtained, which average value inthe case of a correct watermark brought to a correct and correspondingsensor position, is one particular maximum value.

A simple means for obtaining such an average value is a low-pass filter,outlined in FIG. 4 as a resistance R₁ and a capacitance C₁. The voltageU_(DC) is thus a DC voltage representing the average value of U_(ut). Agenuine watermark may be recognized by measuring U_(DC), if the areas 4and 6 of the capacitor plates have been designed properly in accordancewith the shape of the watermark, or in accordance with a characteristicpart of the watermark.

It will be very difficult to bring about a correct DC voltage U_(DC) inany other way than by having a correct watermark coincide with thepattern electrode plates 4 and 6. Security is based upon exactly this,that maximum imbalance between capacitances, which is a necessity forapproval, is obtained only at such a coincidence.

In order to obtain a high degree of security against unwanted influenceby external electrical fields (noise), and to avoid crosstalk betweenthe two successively proceeding capacitance measurements (alternatelyplate 4 and 6), it is advantageous to have each oneshot multivibratorcapacitance input connected to an inside transistor, shown symbolicallyas transistors 19 and 20 in FIG. 6, which is short-circuited to groundduring all of the stable period parts between each unstable interval.Thereby is achieved:

(a) that the part-capacitor which at the moment is not being measured,is grounded, so that only field lines from the presently active platepenetrate the paper and enter the common plate 7. This gives a minimumof crosstalk between the two measurements, since one part-capacitor isheld at a steady potential while the other is charged and vice versa.

(b) that static electricity in the paper is conducted to ground, sincethe note all the time will make contact with ground potential areas onboth sides of the paper.

Another example of a well suited signal processing circuit is shown inFIG. 6. Here the oneshot-multivibrators 16 and 17 are connected inparallel behind a square pulse oscillator 14 which triggers bothmultivibrators at the same time. The duration of the unstable voltagelevel for each one of the multivibrators 16 and C₆, which are connectedto the multivibrators. At the outputs from the multivibrators, which areboth connected to a clock/logic circuit 15, two square pulse trains aregenerated which are equal, i.e. timewise symmetrical, when thecapacitors C₄ and C₆ have a paper of uniform thickness as dielectric,but deviate from each other in time symmetry when the area densitiestake on different values. Examples of curve shapes of the signalsU_(ut4) and U_(ut6) can be found in FIG. 7. A certain degree ofimbalance is shown here, pulse durations being different. The timedifference 2ΔT is timed by the clock/logic circuit 15, which thereaftercompares this value with the desired value which corresponds tocoincidence with a correct watermark.

The oscillator 14 may, if desired, be synchronized to an externalprocess, for example in connection with entering the note into the testarea with the capacitor plates. This is symbolized in FIG. 6 byreference number 18.

The last mentioned measuring method is rapid (within 10-100 μs) becauseof the digital measurement of time differences. However, a certaindegree of crosstalk must be accepted in this case, since both of thecapacitances are measured at the same time and the capacitor plates 4and 6 are situated close by each other and have the counterelectrode 7in common.

It is a common feature of both of said measuring circuits, which areonly working with multivibrators "in phase or counterphase", thatcrosstalk between the two capacitances will not contain very much otherthan the change frequency itself. Thus a stabilization of thecapacitance controlled stop triggering points of the multivibrators aresecured. On the contrary, if the two multivibrators are running freelyrelative to each other, that is with unequal frequencies, there is arisk of superposing for instance a somewhat higher frequency upon thecharge curve of one of the capacitances, giving uncertainty/unstabilityin the stop triggering point.

When the apparatus according to the invention is utilized, the followinghappens:

A note being investigated, is automatically moved into the air gapbetween the electrode plates of the two-part capacitor. In order toobtain maximum correspondence between the possibly correct watermark andthe capacitor pattern, one of a number of well known techniques may beused. As an example, a number of equivalent capacitors may be placed insuccession with a lateral off-set, whereby one of these capacitorsachieves the necessary maximum correspondence, the variation field ofthe watermark position being known for the type of note in question. Or,the note may be moved laterally relative to the capacitor plates inaccordance with a predetermined movement pattern which securescoincidence if the watermark is present. Such techniques are well known,as mentioned above, and do not constitute a part of the presentinvention.

At the moment when the edge of the note reaches the actual area of thecapacitor, a small disturbance of the capacitance balance is obtained,in the opposite direction of the disturbance produced by a correctwatermark, given that the electrode plates of the sensor has afavourable geometric design. When the paper of uniform thickness hasentered the area of the shape adapted electrode plates completely, thecapacitances C₄ and C₆ have been considerably changed due to thepermittivity of the paper, but the symmetry is maintained. In thecircuit variant shown in FIG. 4 the frequency of the square signalU_(ut) decreases, but the DC signal U_(DC) is unchanged, because themean value of U_(ut) is the same.

In the variant shown in FIG. 6 the pulse width of the unstable levelwill change, but equally for both signals. The clock/logic circuit 15thus sees no time difference.

Now, if a forged mark of the imprinted type enters the capacitor area,the shape is correct, but as mentioned previously, the permittivity isabout the same both for thick and thin areas, so that the necessarydegree of assymmetry in capacitance values is not achieved, i.e. themark is not accepted.

When a correct watermark hits the capacitor area, the correct imbalancein the square signal U_(ut) is brought about, and with that the correctDc voltage U_(DC). This correct DC voltage then triggers furthermachinery in order to let the note through, while a rejected note willbe pushed out another outlet in a well known manner per se. Thisreferred to the variant of FIG. 4. Correspondingly a correct timedifference 2ΔT shall occur between the two unstable levels at theoutputs from the multivibrators of FIG. 6, which time difference isinterpreted by the clock/logic circuit as a correct watermark.

It must be remarked that notes with a few wrinkles or small tears do notcause problems for the operation of the device, such defects onlyinfluencing the capacitance to a quite insignificant degree.

It was previously mentioned that it might be advantageous to use only acharacteristic part of the watermark for the measurements. In practice,preferably a watermark section is used which comprises areas of aboutequal sizes of a thinned and a thickened field, even though this is notimperative.

One must underline that the measuring method used in the presentinvention, which is in principle of a static character, entails numerousadvantages. By "a static character" is to be understood that principallythe banknote is lying still, the real capacitance being measured, notonly the capacitance change as the note rushes by. The total capacitanceis for instance related to the note thickness. Thus it will be possibleto deduce the note thickness directly from the sum T₄ +T₆, see FIG. 5.An obvious consequence is that said sum also indicate the occurrence oftwo or more paper notes on top of each other, so that a detection of adouble or multiple feeding is also achieved in the same measurement.

Even if the measurement has a static character, it may be done veryrapidly, adapted to a usual automatic note processing rate. An ordinarybanknote may for instance be tested within less that 0,1 sec., includingentering, positioning and capacitance determining with an indication ofan approval or rejection signal.

A capacitive sensor of the type in question may also be used torecognize an implanted security thread in the paper, the thread beingshaped in a particular way, possibly like a straight line. Thedielectric constant of the security thread is markedly greater than thatof the paper, making it possible to detect the thread with an extendedand adapted electrode shape. The total paper thickness in this area isalso greater than elsewhere. The capacitive sensor may thus beconstructed for detecting both a watermark and a security thread at thesame time.

Arranging two equivalent sensors in sequence, where on is mirrorreversed relative to the other, makes detection of one particular typeof forgery possible, namely a one-side mass addition, for example apiece of tape that is stuck on.

Since the electrical field lines from the shape adjusted electrodes 4and 6 to the grounded common plate 7 do not stand perpendicular to theplates, i.e. the field is not homogenous, the capacitance changes willbe noticeably different when the note is seen effectively from each sidein the respective two measurements. The paper thickness occupiesactually a substantial part of the air gap, and the picture of fieldlines through the added mass is substantially different, depending onwhether this mass is closer to the grounded common plate 7 or the shapeadapted electrode plates 4 and 6.

The following must be remarked about the construction of the practicalapparatus:

In order to minimize noise problems, the grounded common plate 7 or thecapacitor may be connected to a Faraday cage 21, as shown in FIG. 4,enclosing the apparatus. The cage must of course be fitted with thenecessary openings for note entrance and exit. To achieve equalinfluence from temperature variations and external fields on bothmultivibrators, and to avoid stray capacitances, it is preferred to usean integrated circuit with two oneshot-multivibrators built together,and possibly the multivibrators may be formed in a quadruple operationamplifier chip. It is quite important to take care that the assymmetryin the measurements only originates from the capacitances beingmeasured, and not from various external influences. The integratedcircuit is preferably mounted upon the same print card 3 as thepart-plates 4 and 6, in order to minimize wire capacitances.

As mentioned previously, the paper quality may be checked. As the noteenters the sensor, that is before the watermark is in position, U_(ut)in the circuit of FIG. 4 may be used as an indication. An acceptablepaper quality corresponds to a particular sum T₄ +T₆, which may be timedand checked with some suitable, per se known apparatus.

I claim:
 1. A method for approving a document, such as a banknote (1)with a watermark (2a, 2b), the pattern of said watermark consisting oftwo characteristically shaped neighbouring areas (2a, 2b) with a localarea density (mass per unit area) which is markedly both higher andlower than the principal average area density of said banknote (1) inthe watermark region, whereby said watermark or at least acharacteristic section thereof is brought to a position correspondingwith a two-part capacitive sensor device (4, 6, 7), which sensor deviceconsists of a common, flat metal plate (7) as one capacitor side and theother capacitor side is divided into two metal plates (4, 6) situatedboth in a common plane and being electrically separated, withinsignificant separation distance (5) compared to the other areawisedimensions of said two plates (4, 6), and the change in capacitancecaused by the watermark is observed and compared with a change caused bya correct watermark, characterized in that the watermark or saidcharacteristic section thereof is brought in position with a doublyactive capacitive sensor device (4, 6, 7) in which the two plates (4, 6)are situated in a common fixed plane and are adapted in shape to eachone of said two characteristically shaped neighbouring areas (2a, 2b) orsaid characteristic sections thereof, that a preset symmetry property ofthe double output signal from said sensor device is disturbed in apredetermined manner when a correct watermark coincides with the twosensor plates (4, 6), and that the symmetry property is continuouslymonitored by signal processing equipment connected to said sensordevice.
 2. A method as claimed in claim 1, further characterized in thatthe sensor device is arranged in such a way that the capacitancescorresponding to said two metal plates (4, 6) are changed to increaseand decrease respectively, a predetermined amount when an acceptablewatermark is present.
 3. A method as claimed in claim 1 or 2, furthercharacterized in that the sensor capacitances influence circuit means(12, 13) comprised in the signal processing equipment into producing asquare pulse train with a symmetry that is directly related to thecapacitance values, wherein the pulse symmetry or assymmetry is detectedby an average determining circuit (R₁, C₁).
 4. A method as claimed inclaim 3, further characterized in that two "one-shot" multivibrators(12, 13), which are comprised by said circuit means and have theirrespective time constants for the durations of their unstable leveldetermined by each of the sensor capacitances (C₄, C₆), respectively,short circuit their capacitance inputs to ground by means of an internalactive circuit element during every stable period part, whereby themomentarily non-active metal plate (4 or 6) of said other capacitor sideis grounded and whereby static electricity is conducted away from thebanknote.
 5. A method as claimed in claim 4, further characterized inthat the paper thickness, also including a possible occurrence of doubleor multiple banknote feeding, is determined on the basis of one completetime cycle of said square pulse train.
 6. A method as claimed in claim3, further characterized in that the paper thickness, also including apossible occurrence of double or multiple banknote feeding, isdetermined on the basis of one complete time cycle of said square pulsetrain.
 7. A method as claimed in claim 1 or 2, further characterized inthat sensor capacitances (C₄, C₆) influence circuit means (16, 17)comprised in the signal processing equipment into producing two squarepulse trains at separate outputs, with a mutual time symmetry which isdirectly dependent on the capacitance values, wherein time symmetry orassymetry is detected by a clock/logic circuit (15).
 8. Device forapproval of a document, such as a banknote (1) with a watermark (2a,2b), the pattern of said watermark consisting of two characteristicallyshaped neighbouring areas (2a, 2b) with a local area density (mass perunit area) which is markedly both higher and lower than the principalaverage area density of said banknote (1) in the watermark region, thedevice comprising a shape-adapted, two-part capacitive sensor device (4,6, 7) and signal processing equipment connected to the sensor device,said sensor device (4, 6, 7) consisting of one common, flat metal plate(7) on one capacitor side and two metal plates (4, 6) on the othercapacitor side situated both in a common plane and electricallyseparated from each other, with insignificant separation distance (5)compared to the other areawise dimensions of said two plates (4, 6),characterized in that said sensor device (4, 6, 7) is a doubly activecapacitive sensor device, that said two plates (4, 6) are situated in acommon plane and are adapted in shape to each one of said twocharacteristically shaped neighbouring areas (2a, 2b) or characteristicsections thereof and that said signal processing equipment comprisescircuit means (12, 13, R₄, R₆, R₁, C₁) for continuous monitoring of apreset symmetry property of the double output signal from the sensordevice (4, 6, 7).
 9. Device as claimed in claim 8, further characterizedin that said common metal plate (7) is adapted to be connected to agrounded Faraday cage enclosing the whole device, leaving only necessaryopenings for entrance and exit of said note (1).
 10. Device as claimedin claim 8 or 9, further characterized in that said circuit meanscomprise two interconnected "one-shot" multivibrators (12, 13), eachmultivibrator having its time constant determined by appropriateconnections to the respective two parts of said two-part sensor device,said double output signal from said sensor device being defined as theoutput signal (U_(ut)) from one (13) of said multivibrators, wherein theoutput signal may, physical parameters of said circuit means having beenadjusted, have the shape of a symmetrical square signal when the sensordevice detects a region without a watermark, but has its time coursedisturbed in a predetermined manner in the presence of a correctwatermark.
 11. Device as claimed in claim 10, further characterized inthat the capacitance inputs of said multivibrators (12, 13) are adaptedto be short circuited to ground via an internal active circuit elementduring every stable period part.
 12. Device as claimed in claim 11,further characterized in that said one-shot multivibrators areencapsulated in one and the same integrated circuit and mounted close tosaid sensor device, preferably on a common print card (3) comprisingsaid two metal plates (4, 6).
 13. Device as claimed in claim 12, furthercharacterized in that said two metal plates (4, 6) of said sensor deviceadditionally are constructed with a shape adaptation for capacitivedetection of an implanted security thread in the banknote, said securitythread consisting of a metal, metallized plastics, plastics, or asimilar material.
 14. Device as claimed in claim 12, furthercharacterized in that said two metal plates (4, 6) are designed so thatthe sensor device, at the moment when the leading edge of the banknote(1) enters the sensor area, produces a disturbance of balance in theopposite direction of the disturbance produced by a correct watermarkbrought to a coinciding position with said two metal plates (4, 6). 15.Device as claimed in claim 12, further characterized by a further shapeadapted capacitive sensor device, arranged in series behind the firstmentioned sensor device, however with capacitor plates inverted relativeto the plates of the first mentioned sensor device, so that the shapeadapted capacitor plates (4, 6) of the first mentioned sensor device aresituated on one side of the banknote and of the further sensor deviceare situated on the other side of the banknote.
 16. Device as claimed inclaim 11, further characterized in that said circuit means furthercomprise a circuit (R₁, C₁) for determining the average value (U_(DC))of said output signal (U_(ut)).
 17. Device as claimed in claim 16,further characterized in that said two metal plates (4, 6) of saidsensor device additionally are constructed with a shape adaptation forcapacitive detection of an implanted security thread in the banknote,said security thread consisting of a metal, metallized plastics,plastics, or a similar material.
 18. Device as claimed in claim 16,further characterized in that said two metal plates (4, 6) are designedso that the sensor device, at the moment when the leading edge of thebanknote (1) enters the sensor area, produces a disturbance of balancein the opposite direction of the disturbance produced by a correctwatermark brought to a coinciding position with said two metal plates(4, 6).
 19. Device as claimed in claim 16, further characterized by afurther shape adapted capacitive sensor device, arranged in seriesbehind the first mentioned sensor device, however with capacitor platesinverted relative to the plates of the first mentioned sensor device, sothat the shape adapted capacitor plates (4, 6) of the first mentionedsensor device are situated on one side of the banknote and of thefurther sensor device are situated on the other side of the banknote.20. Device as claimed in claim 11, further characterized in that saidtwo metal plates (4, 6) of said sensor device additionally areconstructed with a shape adaptation for capacitive detection of animplanted security thread in the banknote, said security threadconsisting of a metal, metallized plastics, plastics, or a similarmaterial.
 21. Device as claimed in claim 11, further characterized inthat said two metal plates (4, 6) are designed so that the sensordevice, at the moment when the leading edge of the banknote (1) entersthe sensor area, produces a disturbance of balance in the oppositedirection of the disturbance produced by a correct watermark brought toa coinciding position with said two metal plates (4, 6).
 22. Device asclaimed in claim 11, further characterized by a further shape adaptedcapacitive sensor device, arranged in series behind the first mentionedsensor device, however with capacitor plates inverted relative to theplates of the first mentioned sensor device, so that the shape adaptedcapacitor plates (4, 6) of the first mentioned sensor device aresituated on one side of the banknote and of the further sensor deviceare situated on the other side of the banknote.
 23. Device as claimed inclaim 18, further characterized in that said circuit means furthercomprise a circuit (R₁, C₁) for determining the average value (U_(DC))of said output signal (U_(ut)).
 24. Device as claimed in claim 23,further characterized in that said one-shot multivibrators areencapsulated in one and the same integrated circuit and mounted close tosaid sensor device, preferably on a common print card (3) comprisingsaid two metal plates (4, 6).
 25. Device as claimed in claim 24, furthercharacterized in that said two metal plates (4, 6) of said sensor deviceadditionally are constructed with a shape adaptation for capacitivedetection of an implanted security thread in the banknote, said securitythread consisting of a metal, metallized plastics, plastics, or asimilar material.
 26. Device as claimed in claim 24, furthercharacterized in that said two metal plates (4,6) are designed so thatthe sensor device, at the moment when the leading edge of the banknote(1) enters the sensor area, produces a disturbance of balance in theopposite direction of the disturbance produced by a correct watermarkbrought to a coinciding position with said two metal plates (4, 6). 27.Device as claimed in claim 24, further characterized by a further shapeadapted capacitive sensor device, arranged in series behind the firstmentioned sensor device, however with capacitor plates inverted relativeto the plates of the first mentioned sensor device, so that the shapeadapted capacitor plates (4, 6) of the first mentioned sensor device aresituated on one side of the banknote and of the further sensor deviceare situated on the other side of the banknote.
 28. Device as claimed inclaim 23, further characterized in that said two metal plates (4, 6) ofsaid sensor device additionally are constructed with a shape adaptationfor capacitive detection of an implanted security threaded in thebanknote, said security thread consisting of a metal, metallizedplastics, plastics, or a similar material.
 29. Device as claimed inclaim 23, further characterized in that said two metal plates (4, 6) aredesigned so that the sensor device, at the moment when the leading edgeof the banknote (1) enters the sensor area, produces a disturbance ofbalance in the opposite direction of the disturbance produced by acorrect watermark brought to a coinciding position with said two metalplates (4, 6).
 30. Device as claimed in claim 23, further characterizedby a further shape adapted capacitive sensor device, arranged in seriesbehind the first mentioned sensor device, however with capacitor platesinverted relative to the plates of the first mentioned sensor device, sothat the shape adapted capacitor plates (4, 6) of the first mentionedsensor device are situated on one side of the banknote and of thefurther sensor device are situated on the other side of the banknote.31. Device as claimed in claim 10, further characterized in that saidoneshot multivibrator are encapsulated in one and the same integratedcircuit and mounted close to said sensor device, preferably on a commonprint card (3) comprising said two metal plates (4, 6).
 32. Device asclaimed in claim 31, further characterized in that said two metal plates(4, 6) of said sensor device additionally are constructed with a shapeadaptation for capacitive detection of an implanted security thread inthe banknote, said security thread consisting of a metal, metallizedplastics, plastics, or a similar material.
 33. Device as claimed inclaim 31, further characterized in that said two metal plates (4, 6) aredesigned so that the sensor device, at the moment when the leading edgeof the banknote (1) enters the sensor area, produces a disturbance ofbalance in the opposite direction of the disturbance produced by acorrect watermark brought to a coinciding position with said two metalplates (4, 6).
 34. Device as claimed in claim 31, further characterizedby a further shape adapted capacitive sensor device, arranged in seriesbehind the first mentioned sensor device, however with capacitor platesinverted relative to the plates of the first mentioned sensor device, sothat the shape adapted capacitor plates (4, 6) of the first mentionedsensor device are situated on one side of the banknote and of thefurther sensor device are situated on the other side of the banknote.35. Device as claimed in claim 10, further characterized in that saidtwo metal plates (4, 6) of said sensor device additionally areconstructed with a shape adaptation for capacitive detection of animplanted security thread in the banknote, said security threadconsisting of a metal, metallized plastics, plastics, or a similarmaterial.
 36. Device as claimed in claim 10, further characterized inthat said two metal plates (4, 6) are designed to that the sensordevice, at the moment when the leading edge of the banknote (1) entersthe sensor area, produces a disturbance of balance in the oppositedirection of the disturbance produced by a correct watermark brought toa coinciding position with said two metal plates (4, 6).
 37. Device asclaimed in claim 10, further characterized by a further shape adaptedcapacitive sensor device, arranged in series behind the first mentionedsensor device, however with capacitor plates inverted relative to theplates of the first mentioned sensor device, so that the shape adaptedcapacitor plates (4, 6) of the first mentioned sensor device aresituated on one side of the banknote and of the further sensor deviceare situated on the other side of the banknote.
 38. Device as claimed inclaim 8 or 9, further characterized in that said circuit means comprisetwo "one-shot" multivibrators (16, 17) connected in parallel, eachmultivibrator having its time constant determined by appropriateconnections to the respective two parts of said two-part capacitivesensor device, wherein the multivibrators are adapted to be triggeredsynchronously by a square pulse oscillator (14) and to deliver each anoutput signal (U_(ut4), U_(ut6)) to a clock/logic circuit (15) which isadapted to measure the degree of time symmetry or assymmetry between thetwo output signals.
 39. Device as claimed in claim 38, furthercharacterized in that said one-shot multivibrators are encapsulated inone and the same integrated circuit and mounted close to said sensordevice, preferably on a common print card (3) comprising said two metalplates (4, 6).
 40. Device as claimed in claim 39, further characterizedin that said two metal plates (4, 6) of said sensor device additionallyare constructed with a shape adaptation for capacitive detection of animplanted security thread in the banknote, said security threadconsisting of a metal, metallized plastics, plastics, or a similarmaterial.
 41. Device as claimed in claim 39, further characterized inthat said two metal plates (4, 6) are designed so that the sensordevice, at the moment when the leading edge of the banknote (1) entersthe sensor area, produces a disturbance of balance in the oppositedirection of the disturbance produced by a correct watermark brought toa coinciding position with said two metal plates (4, 6).
 42. Device asclaimed in claim 39, further characterized by a further shape adaptedcapacitive sensor device, arranged in series behind the first mentionedsensor device, however with capacitor plates inverted relative to theplates of the first mentioned sensor device, so that the shape adaptedcapacitor plates (4, 6) of the first mentioned sensor device aresituated on one side of the banknote and of the further sensor deviceare situated on the other side of the banknote.
 43. Device as claimed inclaim 38, further characterized in that said two metal plates (4, 6) ofsaid sensor device additionally are constructed with a shape adaptationfor capacitive detection of an implanted security thread in thebanknote, said security thread consisting of a metal, metallizedplastics, plastics, or a similar material.
 44. Device as claimed inclaim 38, further characterized in that said two metal plates (4, 6) aredesigned so that the sensor device, at the moment when the leading edgeof the banknote (1) enters the sensor area, produces a disturbance ofbalance in the opposite direction of the disturbance produced by acorrect watermark brought to a coinciding position with said two metalplates (4, 6).
 45. Device as claimed in claim 38, further characterizedby a further shape adapted capacitive sensor device, arranged in seriesbehind the first mentioned sensor device, however with capacitor platesinverted relative to the plates of the first mentioned sensor device, sothat the shape adapted capacitor plates (4, 6) of the first mentionedsensor device are situated on one side of the banknote and of thefurther sensor device are situated on the other side of the banknote.46. Device as claimed in one of claims 8 or 9, further characterized inthat said two metal plates (4, 6) of said sensor device additionally areconstructed with a shape adaptation for capacitive detection of animplanted security thread in the banknote, said security threadconsisting of a metal, metallized plastics, plastics, or a similarmaterial.
 47. Device as claimed in claim 46, further characterized inthat said two metal plates (4, 6) are designed so that the sensordevice, at the moment when the leading edge of the banknote (1) entersthe sensor area, produces a disturbance of balance in the oppositedirection of the disturbance produced by a correct watermark brought toa coinciding position with said two metal plates (4, 6).
 48. Device asclaimed in one of claims 8 or 9, further characterized in that said twometal plates (4, 6) are designed so that the sensor device, at themoment when the leading edge of the banknote (1) enters the sensor area,produces a disturbance of balance in the opposite direction of thedisturbance produced by a correct watermark brought to coincidingposition with said two metal plates (4, 6).
 49. Device as claimed inclaim 48, further characterized by a further shape adapted capacitivesensor device, arranged in series behind the first mentioned sensordevice, however with capacitor plates inverted relative to the plates ofthe first mentioned sensor device, so that the shape adapted capacitorplates (4, 6) of the first mentioned sensor device are situated on oneside of the banknote and of the further sensor device are situated onthe other side of the banknote.
 50. Device as claimed in claim 8 or 9,further characterized by a further shape adapted capacitive sensordevice, arranged in series behind the first mentioned sensor device,however with capacitor plates inverted relative to the plates of thefirst mentioned sensor device, so that the shape adapted capacitorplates (4, 6) of the first mentioned sensor device are situated on oneside of the banknote and of the further sensor device are situated onthe other side of the banknote.